WATSON AND WHERRY: PYROLUSITE FROM VIRGINIA 551 



tests, rather than estabHshed by accurate chemical analysis, or 

 a combination of the two. 



The chief object of the present study is definitely to establish 

 the mineral identity both of the crystals and crystalline mineral 

 from the Virginia locality, and to direct attention by way of 

 suggestion to the possible bearing on other occurrences. 



Little Fort Valley in which the manganese deposits occur is 

 structurally a synclinal valley composed of Silurian and Devonian 

 sediments. At the Stockwood Realty Corporation's mine where 

 the specimens forming the basis of this study were collected, the 

 manganese oxide ore is associated with Oriskany conglomerate, 

 the quartz pebbles of which range up to half an inch in diameter, 

 and with a fine- to medium-grained sandstone just below, which 

 probably is Salina but may prove upon detailed study to be 

 Oriskany. These are brittle siliceous rocks that have been frac- 

 tured and brecciated in the trough of the syncline where the 

 manganese ore is concentrated. Most of the ore is apparently in 

 the underlying light-colored sandstone, which is brecciated and 

 replaced by manganese oxide. Where impregnation and re- 

 placement of the sandstone by manganese oxide have not reached 

 an advanced stage, the ore is low grade and siliceous. 



The sandstone is nearly white when fresh and not mineralized, 

 composed almost entirely of quartz, and weathers to a light 

 rusty brown. Microscopic examination of the mineralized rock 

 shows, besides quartz and manganese oxide, an occasional zircon, 

 and rather frequent hair-like inclusions of rutile. Much of the 

 quartz exhibits pronounced optical disturbance and often granu- 

 lation. Enlargement of the quartz grains was not observed. 



Wad occurs, but most of the ore observed at the time of my 

 visit was crystalline pyrolusite developed largely by replace- 

 ment of quartz from circulating ground waters, but partly also 

 as a breccia-filling or cement and partly as crusts coating joint 

 surfaces and other spacings in the rocks. 



Replacement of the rock by black manganese oxide (pyrolusite) 

 is plainly shown in hand specimens. All stages of replacement 

 are readily traced in thin sections under the microscope, from the 

 incipient stage in which the quartz grains show only slight 



